Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic d-block element, it has historically been classified as a rare-earth element, together with yttrium and the lanthanides. It was discovered in 1879 by spectral analysis of the minerals euxenite and gadolinite from Scandinavia.
Scandium is present in most of the deposits of rare-earth and uranium compounds, but it is extracted from these ores in only a few mines worldwide. Because of the low availability and the difficulties in the preparation of metallic scandium, which was first done in 1937, applications for scandium were not developed until the 1970s. The positive effects of scandium on aluminium alloys were discovered in the 1970s, and its use in such alloys remains its only major application. The global trade of scandium oxide is about 10 tonnes per year.
The properties of scandium compounds are intermediate between those of aluminium and yttrium. A diagonal relationship exists between the behavior of magnesium and scandium, just as there is between beryllium and aluminium. In the chemical compounds of the elements in group 3, the predominant oxidation state is +3.
|Standard atomic weight Ar, std(Sc)||44.955908(5)|
|Scandium in the periodic table|
|Atomic number (Z)||21|
|Element category||transition metal|
|Electron configuration||[Ar] 3d1 4s2|
Electrons per shell
|2, 8, 9, 2|
|Phase at STP||solid|
|Melting point||1814 K (1541 °C, 2806 °F)|
|Boiling point||3109 K (2836 °C, 5136 °F)|
|Density (near r.t.)||2.985 g/cm3|
|when liquid (at m.p.)||2.80 g/cm3|
|Heat of fusion||14.1 kJ/mol|
|Heat of vaporization||332.7 kJ/mol|
|Molar heat capacity||25.52 J/(mol·K)|
|Oxidation states||+1, +2, +3 (an amphoteric oxide)|
|Electronegativity||Pauling scale: 1.36|
|Atomic radius||empirical: 162 pm|
|Covalent radius||170±7 pm|
|Van der Waals radius||211 pm|
Spectral lines of scandium
|Crystal structure|| hexagonal close-packed (hcp)|
|Thermal expansion||α, poly: 10.2 µm/(m·K) (at r.t.)|
|Thermal conductivity||15.8 W/(m·K)|
|Electrical resistivity||α, poly: 562 nΩ·m (at r.t., calculated)|
|Magnetic susceptibility||+315.0·10−6 cm3/mol (292 K)|
|Young's modulus||74.4 GPa|
|Shear modulus||29.1 GPa|
|Bulk modulus||56.6 GPa|
|Brinell hardness||736–1200 MPa|
|Prediction||Dmitri Mendeleev (1871)|
|Discovery and first isolation||Lars Fredrik Nilson (1879)|
|Main isotopes of scandium|
Scandium is a soft metal with a silvery appearance. It develops a slightly yellowish or pinkish cast when oxidized by air. It is susceptible to weathering and dissolves slowly in most dilute acids. It does not react with a 1:1 mixture of nitric acid (HNO3) and 48% hydrofluoric acid (HF), possibly due to the formation of an impermeable passive layer. Scandium turnings ignite in air with a brilliant yellow flame to form scandium oxide.
In nature, scandium is found exclusively as the isotope 45Sc, which has a nuclear spin of 7/2; this is its only stable isotope. Thirteen radioisotopes have been characterized with the most stable being 46Sc, which has a half-life of 83.8 days; 47Sc, 3.35 days; the positron emitter 44Sc, 4 h; and 48Sc, 43.7 hours. All of the remaining radioactive isotopes have half-lives less than 4 hours, and the majority of these have half-lives less than 2 minutes. This element also has five nuclear isomers, with the most stable being 44mSc (t1/2 = 58.6 h).
The isotopes of scandium range from 36Sc to 60Sc. The primary decay mode at masses lower than the only stable isotope, 45Sc, is electron capture, and the primary mode at masses above it is beta emission. The primary decay products at atomic weights below 45Sc are calcium isotopes and the primary products from higher atomic weights are titanium isotopes.
In Earth's crust, scandium is not rare. Estimates vary from 18 to 25 ppm, which is comparable to the abundance of cobalt (20–30 ppm). Scandium is only the 50th most common element on Earth (35th most abundant in the crust), but it is the 23rd most common element in the Sun. However, scandium is distributed sparsely and occurs in trace amounts in many minerals. Rare minerals from Scandinavia and Madagascar such as thortveitite, euxenite, and gadolinite are the only known concentrated sources of this element. Thortveitite can contain up to 45% of scandium in the form of scandium oxide.
The world production of scandium is in the order of 15 tonnes per year, in the form of scandium oxide. The demand is about 50% higher, and both the production and demand keep increasing. In 2003, only three mines produced scandium: the uranium and iron mines in Zhovti Vody in Ukraine, the rare-earth mines in Bayan Obo, China, and the apatite mines in the Kola peninsula, Russia; since then many other countries have built scandium-producing facilities, including 5 tonnes/year (7.5 tonnes/year Sc2O3) by Nickel Asia Corporation and Sumitomo Metal Mining in the Philippines. In each case scandium is a byproduct from the extraction of other elements and is sold as scandium oxide.
Madagascar and the Iveland-Evje region in Norway have the only deposits of minerals with high scandium content, thortveitite (Sc,Y)2(Si2O7) and kolbeckite ScPO4·2H2O, but these are not being exploited.
The absence of reliable, secure, stable, long-term production has limited the commercial applications of scandium. Despite this low level of use, scandium offers significant benefits. Particularly promising is the strengthening of aluminium alloys with as little as 0.5% scandium. Scandium-stabilized zirconia enjoys a growing market demand for use as a high-efficiency electrolyte in solid oxide fuel cells.
Because of its rarity, scandium is among the most expensive elements. Price for pure scandium fluctuates between 4,000 and 20,000 US dollars per kilogram. Meanwhile, the limited market generates a variety of prices at any given time. In 2010, at the peak of the rare-earths shortage, the price of scandium rose to over 15,000 US dollars per kilogram, and the widely commercially used scandium oxide (Sc2O3) was selling above 7 000 US dollars per kilogram. Since then the limited demand coupled with steady production keeps the price at its 20-year average.
Scandium chemistry is almost completely dominated by the trivalent ion, Sc3+. The radii of M3+ ions in the table below indicate that the chemical properties of scandium ions have more in common with yttrium ions than with aluminium ions. In part because of this similarity, scandium is often classified as a lanthanide-like element.
The halides ScX3, where X= Cl, Br, or I, are very soluble in water, but ScF3 is insoluble. In all four halides, the scandium is 6-coordinated. The halides are Lewis acids; for example, ScF3 dissolves in a solution containing excess fluoride ion to form [ScF6]3−. The coordination number 6 is typical for Sc(III). In the larger Y3+ and La3+ ions, coordination numbers of 8 and 9 are common. Scandium triflate is sometimes used as a Lewis acid catalyst in organic chemistry.
Scandium forms a series of organometallic compounds with cyclopentadienyl ligands (Cp), similar to the behavior of the lanthanides. One example is the chlorine-bridged dimer, [ScCp2Cl]2 and related derivatives of pentamethylcyclopentadienyl ligands.
Compounds that feature scandium in oxidation states other than +3 are rare but well characterized. The blue-black compound CsScCl3 is one of the simplest. This material adopts a sheet-like structure that exhibits extensive bonding between the scandium(II) centers. Scandium hydride is not well understood, although it appears not to be a saline hydride of Sc(II). As is observed for most elements, a diatomic scandium hydride has been observed spectroscopically at high temperatures in the gas phase. Scandium borides and carbides are non-stoichiometric, as is typical for neighboring elements.
Dmitri Mendeleev, who is referred to as the father of the periodic table, predicted the existence of an element ekaboron, with an atomic mass between 40 and 48 in 1869. Lars Fredrik Nilson and his team detected this element in the minerals euxenite and gadolinite in 1879. Nilson prepared 2 grams of scandium oxide of high purity. He named the element scandium, from the Latin Scandia meaning "Scandinavia". Nilson was apparently unaware of Mendeleev's prediction, but Per Teodor Cleve recognized the correspondence and notified Mendeleev.
Metallic scandium was produced for the first time in 1937 by electrolysis of a eutectic mixture of potassium, lithium, and scandium chlorides, at 700–800 °C. The first pound of 99% pure scandium metal was produced in 1960. Production of aluminium alloys began in 1971, following a US patent. Aluminium-scandium alloys were also developed in the USSR.
In early 2018, evidence was gathered from spectrometer data of significant scandium, vanadium and yttrium abundances in red giant stars in the Nuclear Star Cluster (NSC) in the Galactic Center. Further research showed that this was an illusion caused by the relatively low temperature (below 3,500 K) of these stars masking the abundance signals, and that this phenomenon was observable in other red giants.
The addition of scandium to aluminium limits the grain growth in the heat zone of welded aluminium components. This has two beneficial effects: the precipitated Al3Sc forms smaller crystals than in other aluminium alloys, and the volume of precipitate-free zones at the grain boundaries of age-hardening aluminium alloys is reduced. Both of these effects increase the usefulness of the alloy. However, titanium alloys, which are similar in lightness and strength, are cheaper and much more widely used.
The alloy Al20Li20Mg10Sc20Ti30 is as strong as titanium, light as aluminium, and hard as ceramic.
The main application of scandium by weight is in aluminium-scandium alloys for minor aerospace industry components. These alloys contain between 0.1% and 0.5% of scandium. They were used in the Russian military aircraft, specifically the MiG-21 and MiG-29.
Some items of sports equipment, which rely on high-performance materials, have been made with scandium-aluminium alloys, including baseball bats and bicycle frames and components. Lacrosse sticks are also made with scandium. The American firearm manufacturing company Smith & Wesson produces semi-automatic pistols and revolvers with frames of scandium alloy and cylinders of titanium or carbon steel.
Dentists use erbium-chromium-doped yttrium-scandium-gallium garnet (Er,Cr:YSGG) lasers for cavity preparation and in endodontics.
The first scandium-based metal-halide lamps were patented by General Electric and initially made in North America, although they are now produced in all major industrialized countries. Approximately 20 kg of scandium (as Sc2O3) is used annually in the United States for high-intensity discharge lamps. One type of metal-halide lamp, similar to the mercury-vapor lamp, is made from scandium triiodide and sodium iodide. This lamp is a white-light source with high color rendering index that sufficiently resembles sunlight to allow good color-reproduction with TV cameras. About 80 kg of scandium is used in metal-halide lamps/light bulbs globally per year.
Elemental scandium is considered non-toxic, though extensive animal testing of scandium compounds has not been done. The median lethal dose (LD50) levels for scandium chloride for rats have been determined as 4 mg/kg for intraperitoneal and 755 mg/kg for oral administration. In the light of these results, compounds of scandium should be handled as compounds of moderate toxicity.
Aluminium alloys (or aluminum alloys; see spelling differences) are alloys in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, for example rolled plate, foils and extrusions. Cast aluminium alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. The most important cast aluminium alloy system is Al–Si, where the high levels of silicon (4.0–13%) contribute to give good casting characteristics. Aluminium alloys are widely used in engineering structures and components where light weight or corrosion resistance is required.Alloys composed mostly of aluminium have been very important in aerospace manufacturing since the introduction of metal-skinned aircraft. Aluminium-magnesium alloys are both lighter than other aluminium alloys and much less flammable than alloys that contain a very high percentage of magnesium.Aluminium alloy surfaces will develop a white, protective layer of aluminium oxide if left unprotected by anodizing and/or correct painting procedures. In a wet environment, galvanic corrosion can occur when an aluminium alloy is placed in electrical contact with other metals with more positive corrosion potentials than aluminium, and an electrolyte is present that allows ion exchange. Referred to as dissimilar-metal corrosion, this process can occur as exfoliation or as intergranular corrosion. Aluminium alloys can be improperly heat treated. This causes internal element separation, and the metal then corrodes from the inside out.Aluminium alloy compositions are registered with The Aluminum Association. Many organizations publish more specific standards for the manufacture of aluminium alloy, including the Society of Automotive Engineers standards organization, specifically its aerospace standards subgroups, and ASTM International.Group 3 element
Group 3 is a group of elements in the periodic table. This group, like other d-block groups, should contain four elements, but it is not agreed what elements belong in the group. Scandium (Sc) and yttrium (Y) are always included, but the other two spaces are usually occupied by lanthanum (La) and actinium (Ac), or by lutetium (Lu) and lawrencium (Lr); less frequently, it is considered the group should be expanded to 32 elements (with all the lanthanides and actinides included) or contracted to contain only scandium and yttrium. When the group is understood to contain all of the lanthanides, its trivial name is the rare-earth metals.
Three group 3 elements occur naturally: scandium, yttrium, and either lanthanum or lutetium. Lanthanum continues the trend started by two lighter members in general chemical behavior, while lutetium behaves more similarly to yttrium. While the choice of lutetium would be in accordance with the trend for period 6 transition metals to behave more similarly to their upper periodic table neighbors, the choice of lanthanum is in accordance with the trends in the s-block, which the group 3 elements are chemically more similar to. They all are silvery-white metals under standard conditions. The fourth element, either actinium or lawrencium, has only radioactive isotopes. Actinium, which occurs only in trace amounts, continues the trend in chemical behavior for metals that form tripositive ions with a noble gas configuration; synthetic lawrencium is calculated and partially shown to be more similar to lutetium and yttrium. So far, no experiments have been conducted to synthesize any element that could be the next group 3 element. Unbiunium (Ubu), which could be considered a group 3 element if preceded by lanthanum and actinium, might be synthesized in the near future, it being only three spaces away from the current heaviest element known, oganesson.Isotopes of scandium
Naturally occurring scandium (21Sc) is composed of one stable isotope, 45Sc. Twenty-five radioisotopes have been characterized, with the most stable being 46Sc with a half-life of 83.8 days, 47Sc with a half-life of 3.35 days, and 48Sc with a half-life of 43.7 hours and 44Sc with a half-life of 3.97 hours. All the remaining isotopes have half-lives that are less than four hours, and the majority of these have half-lives that are less than two minutes, the least stable being proton unbound 39Sc with a half-life shorter than 300 nanoseconds. This element also has 13 meta states with the most stable being 44m2Sc (t1/2 58.6 h).
The isotopes of scandium range in atomic weight from 38 u (36Sc) to 62 u (62Sc). The primary decay mode at masses lower than the only stable isotope, 45Sc, is beta-plus or electron capture, and the primary mode at masses above it is beta-minus. The primary decay products at atomic weights below 45Sc are calcium isotopes and the primary products from higher atomic weights are titanium isotopes.Lead scandium tantalate
Lead scandium tantalate (PST) is a ferroelectric ceramic material with perovskite structure. It has the formula Pb(ScxTa1−x)O3. The x is usually about 0.5.
Like structurally similar lead zirconate titanate and barium strontium titanate, PST can be used for manufacture of uncooled focal plane array infrared imaging sensors for thermal cameras. Both bulk and thin film structures are used.
It is a mixed oxide of lead, scandium, and tantalum.List of alloys
This is a list of named alloys grouped alphabetically by base metal. Within these headings, the alloys are also grouped alphabetically. Some of the main alloying elements are optionally listed after the alloy names.Organoscandium chemistry
Organoscandium chemistry is the chemistry of organometallic compounds containing a carbon to scandium chemical bond. The interest in organoscandium compounds is mostly academic but several compound classes find practical application in catalysis, especially in polymerization. A common precursor is scandium chloride.
As with the other elements in group 3 – e.g. yttrium, forming organoyttrium compounds – and the lanthanides, the dominant oxidation state for scandium in organometallic compounds is +3 (electron configuration [Ar] 3d14s2). The members of this group also have large ionic radii with vacant s,p and d orbitals (88 pm for Sc3+ compared to 67 pm for Al3+) and as a result they behave as hard Lewis acids and tend to have high coordination numbers of 9 to 12. The metal to ligand chemical bond is largely ionic.Period 4 element
A period 4 element is one of the chemical elements in the fourth row (or period) of the periodic table of the elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The fourth period contains 18 elements, beginning with potassium and ending with krypton. As a rule, period 4 elements fill their 4s shells first, then their 3d and 4p shells, in that order; however, there are exceptions, such as chromium.Scandium(III) hydride
Scandium trihydride is an unstable molecular chemical compound with the chemical formula ScH3. It has been formed as one of a number of other molecular scandium hydride products at low temperature using laser ablation and identified by infrared spectroscopy.
Scandium trihydride has recently been the subject of Dirac–Hartree–Fock relativistic calculation studies, which investigate the stabilities, geometries, and relative energies of hydrides of the formula MH3, MH2, or MH.Scandium(III) sulfide
Scandium(III) sulfide is a chemical compound of scandium and sulfur with the chemical formula Sc2S3. It is a yellow solid.Scandium(III) trifluoromethanesulfonate
Scandium trifluoromethanesulfonate, commonly called scandium triflate, is a chemical compound with formula Sc(SO3CF3)3, a salt consisting of scandium cations Sc3+ and triflate SO3CF3− anions.
Scandium triflate is used as a reagent in organic chemistry as a Lewis acid. Compared to other Lewis acids, this reagent is stable towards water and can often be used in organic reactions as a true catalyst rather than one used in stoichiometric amounts. The compound is prepared by reaction of scandium oxide with trifluoromethanesulfonic acid.
An example of the scientific use of scandium triflate is the Mukaiyama aldol addition reaction between benzaldehyde and the silyl enol ether of cyclohexanone with an 81% yield.Scandium bromide
Scandium bromide, or ScBr3, is a trihalide, hygroscopic, water-soluble chemical compound of scandium and bromine.Scandium chloride
Scandium(III) chloride is the inorganic compound with the formula ScCl3. It is a white, high-melting ionic compound, which is deliquescent and highly water-soluble. Scandium(III) chloride is mainly of interest in the research laboratory. Both the anhydrous form and hexahydrate (ScCl3•6H2O) are commercially available.Scandium dodecaboride
Scandium dodecaboride is a refractory metal boride.Scandium fluoride
Scandium(III) fluoride, ScF3, is an ionic compound. It is slightly soluble in water but dissolves in the presence of excess fluoride to form the ScF63− anion.Scandium hydride
Scandium hydride, also known as scandium–hydrogen alloy, is an alloy made by combining scandium and hydrogen. Hydrogen acts as a hardening agent, preventing dislocations in the scandium atom crystal lattice from sliding past one another. Varying the amount of hydrogen controls qualities such as the hardness of the resulting scandium hydride. Scandium hydride with increased hydrogen content can be made harder than scandium.
It can be formed by progressive hydrogenation of scandium foil with hydrogen.In the narrow range of concentrations which make up scandium hydride, mixtures of hydrogen and scandium can form two different structures. At room temperature, the most stable form of scandium is the hexagonal close-packed (HCP) structure α-scandium. It is a fairly soft metallic material that can dissolve a moderate concentration of hydrogen, no more than 0.89 wt% at 22 °C. If scandium hydride contains more than 0.89% hydrogen at room temperature then it transforms into a face-centred cubic (FCC) structure, the δ-phase. It can dissolve considerably more hydrogen, as much as 4.29%, which reflects the upper hydrogen content of scandium hydride.
Research indicates the existence of a third phase created under extreme conditions termed the η-phase. This phase can dissolve as much as 6.30% hydrogen.
Concentration dependent activation-energies are observed for hydrogen diffusion in scandium metal.Scandium nitrate
Scandium(III) nitrate, Sc(NO3)3, is an ionic compound. It is an oxidizer, as all nitrates are. It is applied in optical coatings, catalysts, electronic ceramics and the laser industry.Scandium oxide
Scandium(III) oxide, Sc2O3, or scandia, is a high melting rare earth oxide. It is used in the preparation of other scandium compounds as well as in high-temperature systems (for its resistance to heat and thermal shock), electronic ceramics, and glass composition (as a helper material).Scandium sulfate
Scandium sulfate is the scandium salt of sulfuric acid and has the formula Sc2(SO4)3. It is used in agriculture as a very dilute solution as a seed treatment to improve the germination of corn, peas, wheat, and other plants.